The diffusers most frequently used in power gas turbines are known to be derived from those designed and arranged for aeronautical turbines in which a small overall radial dimension is essential, and in which the diffusion duct must be traversed by aerodynamically profiled double-wall ribs, which are cooled in the interspace with cold gas for supporting the shaft bearing which would otherwise not be reachable.
These diffusers are compromised of two coaxial conical walls with an angle of about 7.degree. between the cones. In this respect, a diffuser of this type has its maximum efficiency under conditions of best compromise between the friction loss at the two walls, which is dependent on length for equal surface finishes and is thus smaller the shorter the diffuser, and the diffusion turbulence losses which are smaller the more gradual the diffusion and thus the longer the diffuser. It has been found experimentally that the optimum compromise length, dependent on degree of finish, velocity etc., corresponds as a first approximation to an angle between the cones of about 7.degree. for two-wall diffusers of prevalently axial extension.
On the other hand, the gas is still at high velocity when leaving the diffuser, and its energy is therefore lost, but as the exhaust is axial and taking into account the aeronautical compromise between weight, overall size and efficiency, this loss is accepted.
Land-based turbines, which derive from aeronautical experience, use similar diffusers, the only difference being that at the end of the diffuser the gas is made to curve into the radial direction due to the fact that the exhaust is radial in land-based turbines. In order to curve the gas with smaller losses and smaller radii, formations of deflectors are often arranged in the bend, and have a cross-section in the form of parallel circular arcs. Gas diffusion is considered finished at the end of the conical portion, and the deflectors serve only to reduce the pressure drop through the bend, and not for diffusion purposes.
Land-based turbines derived from aeronautical technology do not exploit the larger range of alternatives offered by land installations over aeronautical installations for the following reasons:
a) they retain the bearing support ribs at the diffuser inlet where the gas is of considerable velocity, resulting in a certain loss which becomes much greater if the turbine has to operate under other than design conditions. In such a case the loss caused by the reduction in cross-section during passage through the ribs is supplemented by the loss caused by the impact of the gas against the ribs, this impact occurring at an angle of incidence which is more removed from the optimum angle the more the operation deviates from the design point (in the case of land-based turbines, it is not unusual to have to operate at 50% of the initial design speed). In the aeronautical turbine, the ribs are essential for overall size and weight reasons. In the land-based turbines, the bearing could instead be supported from the outside if certain mechanical problems related to the shaft line are solved; and
b) they do not reduce the exhaust gas velocity to a minimum without negative effects in the efficiency and noise level.
One type of diffuser which is beginning to be adopted in land-based turbines is precisely characterized by the elimination of these ribs and an attempt to improve diffusion in the final bend. The ribs are eliminated by supporting the bearing from the outside, given that the exhaust is no longer axial, and the bend is made in the form of a truly curved diffuser which is much more complicated than a straight diffuser but which by careful design and experimental setting-up can attain a further worthwhile recovery.
In order to further improve this type of diffuser, it is necessary either to increase the axial conical portion so as to arrive at the bend with a greater diffusion ratio, which however causes an intolerable increase in the axial turbine length, or to dispose an intermediate wall in said portion so as to double the diffusion angle. This path has been followed in particular by those manufacturers who retain the bearing support ribs so as to also support the intermediate wall by these latter. However, this design gives only insignificant results for obvious reasons. In this respect, by retaining the ribs, all the aforesaid losses under working conditions other than the design condition still occur, and in addition because of the said balance between friction losses and diffusion losses, the introduction of the double wall into the zone in which the gas is still at high speed leads to an increase in losses due to friction and entry impact, which strongly reduce the theoretical advantages of the increased diffusion.
A second path would be to increase the curved diffuser portion, but this would lead to an increased overall radial dimension which in the case of large turbines is even less desirable (transport problems etc.).